Syntheses and Crystal Structures of New Vanadium(IV) Oxyphosphates M(VO)2(PO4)2 with M: Co, Ni.

Abstract We have extended our research interest on titanium oxyphosphates (M II (TiO) 2 (PO 4 ) 2 , with M II  = Mg, Fe, Co, Ni, Cu, Zn) to vanadium oxyphosphates M II (V IV O) 2 (PO 4 ) 2 (M II  = Co, Ni). For each compound two phases, named α and β according to synthesis conditions, have been stabilized at room temperature, then characterized. The four crystal structures M(VO) 2 (PO 4 ) 2 (α and β for M = Co, Ni) have been determined in monoclinic P 2 1 / c space group using X-ray single crystals diffraction data. Structure of the α phase is derived from the Li(TiO)(PO 4 ) (orthorhombic Pnma ) and LiNi 0.50 (TiO) 2 (PO 4 ) 2 (monoclinic P 2 1 / c ) types, with cell parameters: a  = 6.310(1) A, b  = 7.273(1) A, c  = 7.432(1) A, β  = 90.43(1)° for M = Co, and a  = 6.297(2) A, b  = 7.230(2) A, c  = 7.421(2) A, β  = 90.36(2)° for M = Ni. Structure of the β phase is derived from the Ni(TiO) 2 (PO 4 ) 2 -type (monoclinic P 2 1 / c ) with cell parameters: a  = 7.2742(2) A, b  = 7.2802(2) A, c  = 7.4550(2) A, β  = 120.171(2)° for M = Co, and a  = 7.2691(2) A, b  = 7.2366(2) A, c  = 7.4453(2) A, β  = 120.231(2)° for M = Ni. All these structures consist of a three dimensional (3D) framework built up of infinite chains of tilted corner-sharing [VO 6 ] octahedra, cross-linked by corner-sharing [PO 4 ] tetrahedra. The M 2+ ion (M = Co, Ni) is located in a triangular based antiprism which shares faces with two [VO 6 ] octahedra. Structural filiation is discussed based on a common structural unit, a sheet where divalent cations M 2+ (M = Co, Ni) are inserted. A thermal study of the α ↔ β transition is also presented.